Update CHANGELOG for 0.2.8
Update CHANGELOG for 0.2.8

wantedly/step-pretty-slack-notify,dlanileonardo/step-pretty-slack-notify,dlanileonardo/step-pretty-slack-notify,wantedly/step-pretty-slack-notify,wantedly/step-pretty-slack-notify,dlanileonardo/step-pretty-slack-notify